The present invention relates generally to a seal assembly, and more particularly to a seal assembly including at least one seal tooth having at least one hole to reduce swirl-induced rotordynamic instability.
In rotary machines such as turbines, seals are provided between rotating and stationary components. For example, in steam turbines, it is customary to provide a plurality of arcuate packing ring segments to form an annular labyrinth seal between the stationary and rotating components. Typically, the arcuate packing ring segments (typically, four to six per annular seal) are disposed in an annular groove in the stationary component concentric to the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth type seals, a plurality of axially spaced, circumferentially extending seal teeth extend from the stationary component toward the rotating component. The sealing function is achieved by creating turbulent or flow restriction of an operative fluid, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.
In operation, with high rotor rotational velocity, fluid axially entering the fluid path of a rotary machine can acquire a significant tangential velocity component (also called “steam swirl”). For example, as the fluid moves through the labyrinth seal, the fluid may flow between the axially spaced seal teeth and circumferentially around the rotating component. This causes the fluid to acquire the significant tangential velocity component, which can induce rotor instabilities in turbomachines. The magnitude of this rotor instability is a function of the circumferential flow component of fluid within the labyrinth seal.
As more and tighter seals are used in steam turbines, swirl-induced rotordynamic instability becomes more critical for large steam applications. Conventional anti-swirl teeth take up additional axial space and are not rub-friendly, because the clearance has to be set large enough to avoiding rubbing against a rotor generating a lot of heat. To reduce the risk of scoring the rotor, conventional seal segments with anti-swirl features are typically assembled into a stationary component with a spring element to allow the seal ring to move away from the stationary component in case of rotor rubbing.